When manufacturing aluminium fluoride from aluminium hydroxide and fluosilicic acid, several troublesome waste products are obtained which must be deposited and, optionally, rendered harmless. Thus, there is obtained a silica waste which contains fluorine and aluminium impurities, and a mother liquor and a scrubber water containing fluorine, aluminium and silicon. These waste products present a serious problem, since they occur in relatively large quantities and require expensive treatment before they can be deposited. It is true that silica residues per se can be deposited, but--as will be understood--the direct deposit of silica contaminated with large quantities of harmful impurities cannot be tolerated. The mother liquor and the scrubber water must be neutralized with calcium hydroxide to a pH-value of about 10, at which the aluminium, fluorine, and silicon contents thereof precipitate. This precipitate is then separated from the remaining aqueous solution for example by centrifugation, and subsequently deposited.
Thus, the aforementioned waste products constitute a serious environmental problem, but can be rendered harmless, however, to high costs. In addition, substantial amounts of silicon, aluminium, and fluorine are lost when the waste is treated in said manner.
The reactions involved in the production of aluminium fluoride are given below, the waste products being underlined. ##STR1##
Prior art recognizes some different methods for recovering active silica and cryolite as well as other compounds from fluosilicic acid and waste products from the preparation of aluminium fluoride.
Using fluosilicic acid and ammonia as starting materials active silica and cryolite can be prepared (AT, B, 315128). Thereby ammonium fluoride and silica are primarily prepared, whereafter the ammonium fluoride is reacted with sodium aluminate to the formation of cryolite.
SE,B, No. 366723 relates to a process to recover fluorine and aluminium from dilute, aqueous solutions, particularly mother liquors obtained in the crystallization of aluminium fluoride, whereby the mother liquors are treated with ammonia, or ammonium dicarbonate to give a chiolite-analogous ammonium compound, (NH.sub.4).sub.5 Al.sub.3 F.sub.14). The precipitation of said compound is carried out at pH 6-6.5.
SE,B No. 7502835-7 relates to a concentrated synthetic fluxing agent based on calcium fluoride, where remaining fluorine quantities from the production of aluminium fluoride from fluosilicic acid and aluminium hydroxide are precipitated using a calcium compound.
In accordance with SE,A, No. 8100218-0 a method for recovering useful products from waste of the described form is disclosed, whereby silica obtained in the manufacture of aluminium fluoride and contaminated with, inter alia, fluorine and aluminum, is dissolved in a strongly basic hydroxide; mixing the first solution obtained with a second solution obtained by dissolving aluminium hydroxide with a strongly basic hydroxide, and with waste mother liquor and optionally also scrubber water from the manufacture of aluminium fluoride in such proportions that the pH-value of the mixture lies between about 10 and 14, the silica content of the waste products supplied being precipitated as a silicoaluminate, which is separated off, preferably by filtration, for optional further treatment or for direct use, whereafter fluorine, if present in the waste products, is recovered from the filtrate by adding thereto an aluminium compound in an amount sufficient to precipitate substantially all the fluorine content contained in the filtrate as a fluoroaluminate, which is separated off, preferably by filtration, for further use, and whereafter the final filtrate is passed to a recipient or utilized, for example, as process water in other processes.
Another method for recovering useful products from waste of the described type is disclosed in SE,A No. 8200045-6, whereby alkali metal ions are added in the form of an alkali metal salt to collected mother liquors and scrubber water obtained in the manufacture of aluminium fluoride and being contaminated with aluminium, silica, and fluorine, and by adjusting the hydrogen ion concentration of said mother liquors and scrubber water to a pH of 2.0 to 5.0, preferably 2.0 to 3.0 by adding an alkali metal base while maintaining a temperature of said collected solutions of above about 50.degree. C., isolating the alkali metal fluoroaluminate thereby precipitated by filtration, whereafter the filtrate comprising silica and minor amounts of aluminium and fluorine is passed to a second step for the recovery of one or more of said compounds.
Thus, the useful product produced in the first stage is a fluoroaluminate, preferably a sodium fluoroaluminate also named cryolite. This means that in a second step the silica contents of the waste products, i.e. precipitated SiO.sub.2, according to formula I page 2, and the filtrate from step one above, can be recovered in the form of a silicoaluminate, preferably sodium silicoaluminate, thereby the silica is primarily dissolved and reacted with sodium hydroxide to sodium silicate and reacted with an aluminium salt. Such products as sodium fluoroaluminate (cryolite), and sodium silicoaluminate can be used directly, and are requested by the market. Thus, sodium silicoaluminate can be used as a pigment agent in the paper-making industry and in the rubber and paint industries. Sodium fluoroaluminate or cryolite is mainly used as a fluxing agent when producing aluminium electrolytically.
Under normal conditions, however, it has turned out to be too high contaminations of aluminium and particularly fluorine in the silica waste used for the production of silicoaluminates, whereby it has been requested to eliminate these impurities from the silica waste prior to the use of the silica, the contamination of aluminium i.a. being detrimental to the production of waterglass from silica.
It has also been shown that when using sodium silicoaluminate as a pigment agent in the production of paper the residual fluorine content has a great impact on conventional sizing, determined as the Cobb-value, whereby a fluorine content of 2% in the pigment gave a Cobb-value of 93 and a fluorine content of 1% gave a Cobb-value of 45 with the same wood pulp composition, i.e. a more than 50% increase in that range.
Further a content of fluorine present in the silica when producing a sodium silicate from said silica and sodium hydroxide, will render the latter reaction hard to control.
Carrying out the method according to SE No. 8100218-0 a sodium aluminium silicate is obtained which is rather alkaline and will thereby require a high demand of alum when used as a paper filler.
In accordance with SE,A, No. 8200045-6 purification can be made by suspending the silica waste in sulphuric acid, heating the mixture, whereby fluorine and silicofluoride are driven off, absorbed and passed on to the precipitation step of fluoroaluminate, and the silica is suspended in water for dissolving any aluminium sulphate present. The slurry thus obtained is then passed to a filtration step, where the solid and now pure silica is separated off and passed on to the dissolver for production of sodium silicate and the aluminium sulphate solution containing sulphuric acid is passed to the precipitation step of sodium aluminium silicate.
This method for purification is, although convenient, not so far reaching as requested in order to eliminate the fluorine and aluminium contents. Moreover this method is very energy consuming and costly with regard to apparatuses involved.